Imaging system employing ion-permeable control member

ABSTRACT

In the formation of visible copies of an optical image, the use of a conductive ion permeable member coated with a photoconductor which is employed to form an ion image corresponding to an optical image focused upon said ion permeable member. This ion image is caused to impinge upon a fine mesh screen which has been coated with neutral electrostatic toner particles, thereby forming a charged developer particle image and this charged developer particle image is then electrostatically attracted onto the surface of a plain paper receptor sheet.

United States Patent 1191 Fotland et al.

1 1 IMAGING SYSTEM EMPLOYING ION-PERMEABLE CONTROL MEMBER Inventors:Richard A. Fotland, Warrensville Heights; Virgil E. Straughan, Euclid,both of Ohio Horizons Incorporated, a Division of Horisons ResearchIncorporated, Cleveland, Ohio Filed: Aug. 14, 1972 Appl. No.: 279,205

Related US. Application Data Continuation-impart of Ser. Nos. 178,521,Aug. 27, 1971, Pat. No. 3,797,926, and Ser. No, 275,674, Aug. 2, 1972,Pat. No. 3,761,173,

Assignee:

US. Cl. 355/3 DD, 96/1.3, 118/637 Int. Cl..; 603g 15/00 Field of Search355/3 R, 16, 3 DD, 17;

96/1 R, l, 3; 346/74 ES; 118/637 References Cited UNITED STATES PATENTSYork 118/637 3,610,205 10/1971 Rarey et a1. 118/637 3,613,638 10/1971Solarek 118/637 3,645,614 2/1972 McFarlane et a1 355/3 3,680,954 8/1972Frank 355/3 FOREIGN PATENTS OR APPLICATIONS 1,156,308 10/1963 Germany96/1 R OTHER PUBLICATIONS Defensive Publication, T890,003, J. Y.Kaukeinen, Sept. 1971, Latitude in Photocond. Controlled CoronaCharging.

Primary Examiner-Robert P. Greiner Attorney, Agent, or Firm-Lawrence I.Field [5 7] ABSTRACT In the formation of visible copies of an opticalimage, the use of a conductive ion permeable member coated with aphotoconductor which is employed to form an ion image corresponding toan optical image focused upon said ion permeable member. This ion imageis caused to impinge upon a fine mesh screen which has been coated withneutral electrostatic toner particles, thereby forming a chargeddeveloper particle image and this charged developer particle image isthen electrostatically attracted onto the surface of a plain paperreceptor sheet.

5 Claims, 7 Drawing Figures v r 1 I IMAGING SYSTEM EMPLOYINGION-PERMEABLE CONTROL MEMBER more particularly to a method and apparatusfor the efficient formation of electrostatically developed ion currentpatterns corresponding to an optical image.

In conventional plain paper electrostatic photography, an insulatingphotoconductor is charged with a corona source of ions, exposed, thecharge image developed with a toner, the developed toner imagetransferred to plain paper, and finally, the toned image is fixed,usually by fusing. After the transfer operation,

the residual image is erased'from the surface of the photoconductor andthe photoconductor is cleaned in preparation of a repetition of theprocess. Although employing plain paper, this process is complicated bythe requirement fora number of different machine operations. Inaddition, the photoconductor suffers wear over a period of time, sincethe surface of the photoconductor 'is' repeatedly rubbed by tonerparticles, cleaning brushes and paper surfaces.

' A related process employs a photoconductively coated conducting paper.The photoconductor, usually zinc oxide (although organic photoconductorsmay be employed), is first charged, then exposed, and the charge imageis then toned. Here the photoconductor is not reusable and thus the wearand tear restrictions in the aforementioned process are eliminated. Inaddition, the machine operation is simplified. One disadvantage of thisprocess is the requirement for coating the paper with a photoconductor.These photoconductively coated papers are significantly more expensivethan plain uncoated paper. In addition, because of the heavyphotoconductor coating (the coating weight generally amounting to poundsper 3,000 ft ream), the papers are heavy and have a feel quite differentfrom plain paper. I

A principal object of the present invention is to simplify theconventional plain paper electrophotographic process and the apparatusby which it is carried out.'

Another object of the invention is to provide an image reproductionmethod wherein there is no physical contact of the photoconductor witheither developer or paper.

In addition to having the advantages of eliminating photoconductor wearand simplifying the number of machine operations, the method andapparatus of this invention do not require a paper coated with aphotoconductor. In comparison therefore to electrostatic copy processesemploying photoconductive paper, the process of this inventionhas theadvantage of lower paper cost and the advantage of a capability foremploying plain (nonchargeable) paper.

Another object of the invention is to provide an image copying means forgenerating full color copy.

In the present invention, a fine mesh screen or grid uniquely coatedwith a photoconductor is employed to spatially modulate the flow ofcorona current in accordance with an optical image projected onto saidfine mesh screen or grid.

preparing electrostatic images on an image receptor The ion-permeablearray which serves to modulate the flow of ions in accordance with anoptical image is described in detail in co-pending United StatesApplication Ser. No. 178,52l filed Aug. 27, 1971, of which the presentapplication is a continuation in part and in United States ApplicationSer. No. 275,674 filed Aug. 2, 1972.

The method. of utilizing the photoconductor coated screen for theformation of visible images in the practice of the present inventionwill be more fully apparent from the description which follows takenwith the drawings in which:

FIG. 1 is a schematic'view of an apparatus for preparing electrostaticcharge images corresponding to an optical image projected upon an imagereceptive surface.

FIG. 2 and 3 illustrate various apparatus including 'photoconductivelycoated screens in copy operations wherein the final image is formed uponplain paper, that is, paper which is not capable of sustaining a chargeimage. FIG. 2 employs a simultaneous charging and developing operationusing a liquid or dry aerosol, while FIG. 3 employs liquid development,the charging anddevelopment once again being carried out simultaneously.

FIGS. 4, 5 and 6 schematically depict modifications of the device ofFIG. 1 by means permitting simultaneously exposing, charging, anddeveloping a visible image upon plain paper, i.e., non-chargeablemembers, and utilizing an ion current modulating screen.

FIG. 7 is a schematic illustrating a means for generating full colorcopies of an original in a manner which eliminates registrationproblems.

Referring now to FIG. 1, illustrating an apparatus for surface, theapparatus comprisesan electrically conductive platen 10 upon which issupported a conducting paper 12, having a thin dielectric coating 14. Acorona modulating screen, grid or aperture plate 16 controls the ioncurrent reaching the surface of the dielectric paper in accordance withan optical image projected onto elementl6. A corona source is provided,which may comprise a fine wire 18.. The corona operating potential issupplied by power supply 20. The paper support substrate 10 ismaintained at a selected potential provided by power supply 21.Electronic controls 24 provide a means for simultaneously turning onpower supplies 20, 21 and an illumination source for a projector 22.Projector 22 provides the image which is to be copied; this image beingfocused upon screen 16.

Although in this embodiment the optical image is provided by a projectorsuch as might be employed in the projection of microfilm images toobtain hard copy, it will be understood that projector 22 could bereplaced by a cathode-ray tube display using a projection lens system orby an original document support plus a projection system forconventional office copy, or any other suitable source of optical imagedepending upon the application of the apparatus. It should also beunderstood that, although the examples herein refer to an optical(light) exposure, the input to the screen may consist of other forms ofenergy to which the photoconductor employed exhibits sensitivity. Theseother radiations include x-rays, gamma rays, and alpha and betaparticles.

A single corona wire 18 is shown in FIG. 1. In order to provide auniform corona over a large area, a plurality of corona wires may beutilized all connected in parallel to power supply 20. In order toprovide sufficient corona current, the corona wire diameter should beless than 10 mils and to simplify handling of the wire, the wirediameter should be greater than 1 mil. A preferable wire diameter forthis embodiment is 2 mils. Using a single corona wire spacedapproximately 1 inch above modulating screen 16, uniform charging, inaccordance with the projected optical image, of the dielectric paperoccurs over an area equal to the length of the corona wire and adistance between 1 and 2 inches normal to the direction of the coronawire at the paper. In order to provide for more uniform charging, thecorona wire(s) may be moved, in a plane parallel to the screen, duringthe exposure.

A dielectric paper is shown in FIG. 1, such papers being available froma variety of paper mills and being employed widely in high speedcomputer printers and recorders. The dielectric coated paper may bereplaced with any of a number of plastic films ranging in thickness from0.1 to 5 mils. Images have been successfully formed on both polyesterand acetate films; and, indeed, any film which has a dielectricrelaxation time in excess of a few seconds and which falls within theaforementioned. thickness range may be employed in the apparatus of FIG.1.

Means for mechanically transporting the dielectric paper or plastic filmunder the corona modulating screen, maintaining said paper (film)stationary during the exposure, and then removing the paper from theimaging station are not shown in FIG. 1; these mechanical features beingwell known to those skilled in the art. FIG. 1 shows the coronamodulating screen maintained at ground potential. In this event, thepotential on the corona wire and backing plate must be opposite inpolarity. Thus, ifthe corona wire is maintained at a positive potential,the backing plate must be maintained at a negative potential so thatpositive ions emitted from the corona wire are accelerated to thedielectric paper after passing through the meshes of screen 16.Alternately, the backing plate 10 might be maintained at groundpotential, screen 16 ata positive potential, and corona wire 18 at aneven higher positive potential.

The potential required between corona wire 18 and screen 16 must be atleast sufficient to initiate a corona current, i.e., at least 4 to 5 kv.The higher the potential the greater the ion current and hence the morerapidly dielectric paper may be charged and the lower the requiredexposure time. The upper limit of corona potential is realized whensparking occurs between corona wire 18 and screen 16. This is, ofcourse, a function of the spacing between 16 and 18. Corona potentialsas high as 25 kv have been employed in this invention successfully.

The potential required between screen 16 and backing plate 10 dependsupon the spacing between said members and the required resolution'of theelectrostatic image formed on the charge supporting member. If thepotential for a given spacing is too high, sparking will occur betweenthe chargeable member and screen 16. Furthermore, at high potentials fora given spacing, the resolution of the charge image is sufficiently highso that a screen pattern corresponding to the screen 16 is observed inthe charge pattern laid down on the chargeable member. A preferredelectric field, in this region, is 20 kv per inch. This corresponds toan applied potential of 10 kv at a one-half inch spacing or 1 kv at a 50mil spacing. At this electric field the corona current passing throughscreen 16 and onto the chargeable member follows the field linesufficiently well so that a resolution of 6 to 10 line-pairs/mm isreadily obtained with screens having from 240 to 400 meshes per inch. Atelectric fields in the range of 50 to kv per inch, sparking occasionallyoccurs and the screen mesh pattern appears in the image. At fields belowapproximately 3 kv per inch, ion spreading is observed with subsequentdegradation of image resolution.

The exposure times required are a complicated function of the coronavoltage, corona-to-screen spacing, light intensity at the screen, natureof the photoconductor, and also the nature of the charge receivingmember and the type of development employed in converting theelectrostatic image into a visible image. In general, the requiredscreen illumination ranges from 5 to 50 foot-candles of tunstenillumination and the exposure times range from 0.1 to 3 seconds.

Although a preferred means of carrying out the teachings of the presentinvention, as shown in FIG. 1, is superficially similar to the apparatusdescribed in Snelling US. Pat. No. 3,220,324, a number of very importantdifferences exist. The disclosure in Snelling requires that the opticalimage incident upon the corona modulating screen be placed on the sideof the screen opposite the source of corona. Thus, in a majority ofSnellings examples, the optical image is presented to the screen eitherby transmission of said image through a transparent or translucentrecording member or by) reflection from the surface of the recordingmember onto the side of the screen opposite the corona charging member.In the present invention, the optical image is projected onto thescreen, on the same side of the screen as the source of ion current. Itmakes no difference whether the recording member is of high reflectivityor not, and at the large spacings employed with the present inventionthe influence of any reflected light would be to degrade resolutionbecause of this relatively large spacing between the recording memberand the corona modulating member. In addition, the teachings of Snellingindicate a requirement for employing low potentials, on the order of 100to volts, between the recording member conducting backing and the ioncurrent modulating screen. The spacings between these members are alsoindicated as being relatively small (one-sixteenth inch' or less) inorder to preserve high resolution in the electrostatic latent image.Snelling further indicates that hisinvention appears to function becauseof an ion field component buildup between the ion modulating screen andrecording member. In the present invention, the screen serves tomodulate the flow of ions through the screen independently of any changein electric field between the screen and the recording member. Thismodulation permits high potentials and large spacings to be employedbetween the ion modulating member and the recording member.

British Patent Specification Nos. 1,149,901 and 1,152,308 which appearto correspond to US. Pat. No. 3,680,954 also describe the use of anion-permeable screen with an optical image projected thereon to con-'trol the deposition of ions onto a chargeable recording member.Significant differences also exist between the inventions of thesespecifications and the method of employing the photoconductive coatedscreen which is the subject of the present invention. For example, inthe apparatus described in these British specifications, coverage of theconductive grid with a photoconductive material is required to becomplete, even microscopic cracks in the coating are said to bedetrimental to the operation of the apparatus. For this reason, theBritish specifications indicate that fine woven wire mesh is notsatisfactory as a support material for the photoconductive coatingbecause of difficulties observed in completely covering the wires wherethey cross over each other. In the invention described herein, a finewoven wire mesh is one preferred conducting photoconductive support andwith the asymmetrically coated screens of the present invention, actualbare screen is usually present.

The photoconductively coated screen in the British specification isoperated in such a position as to be quite closely adjacent to thecharge supporting recording member, it being indicated that spacing isnot critical if it is within one hole diameter, and elsewhere that thespacing may be 0.1 mm or less.

In contrast, in the practice of the present invention, spacings ofdistances as great as 1 inch may be used, provided high potentials aremaintained between the screen and the latest image receptor sheetconductive backing. In the description in the British specification itis indicated that it is preferable to employ potentials such that theconductive screen and the image receptor sheet conducting backing are atthe same potential or even back biasing, i.e., maintaining the recordingmember potential at some value between the potential of thephotoconductively coated screen and the corona source. It is furtherstated that the potential on the image receptor surface is limited tothe maximum potential which may be sustained across the photoconductor.In British Patent Specification No. 1,152,308, provision is made forspacing the screen further away from the latent ion image recordingmember. Here, however, a second conducting screen must be interposedbetween the photoconductive screen and the recording member in such amanner that the second conducting grid is spaced very closely to thephotoconductively coated screen. The closely spaced conducting screen isagain back biased with respect to the photoconductively coated screen.From this discussion it will be apparent that the present invention issignificantly different from the prior art taught in the Britishspecifications.

Recently issued U.S. PATS. Nos. of Burdidge (3,582,206). McFarlin et al.(3,645,614) and Pressman et al. (3,647,29l all disclose screenscontaining an insulating layer for controlling the flow of ions orcharged particles through the screen. In each of these patents, however,the method of operation involves first charging the screen on one side,next exposing the charged screen to an optical image, and finallyprojecting ions or charged particles through the screen but directedfrom-the opposite side. Thus, the method of employing the screen inthese patents differs significantly from the practices of the presentinvention.

FIG. 2 is a schematic drawing of an apparatus for simultaneouslycharging, exposing and developing. The apparatus shown in FIG. 2 may bethe apparatus in FIG. 1 with the addition thereto of means for injectingan aerosol into the region between corona modulating screen 16 and apaper image receptor sheet 81. The

image receptor sheet 81 in this apparatus does not require a dielectriccoating upon its surface. In order that uniform development occur, it isnecessary that the development aerosol be injected with a high degree ofuniformity into the region between screen 16 and receptor sheet 81. Theair velocity of injection cannot be too high or a displacement andbreakup of the image occurs. In addition, the aerosol must be initiallyuncharged or, if the aerosol particles are charged, the charge must beadjusted to some low value in orderto minimize background. The chargepotential of the aerosol may be controlled within certain limits byadjusting the potential of the conducting manifold from which theparticles are ejected. This is accomplished with a power supply 83, asshown in FIG. 2. Or else, the particle charge may be controlled byinduction, in which case the potential of power supply 83 is notconnected directly to the conducting manifold, but is rather connectedto an electrode immediately adjacent to the aperture or slit in theaerosol generating nozzle 82.

In operation, potentials are applied to appropriate electrodes, theimage is projected onto the corona modulating screen, and the aerosol isinjected into the region between screen and receptor sheet all processesoccurring simultaneously. The aerosol particles, being essentiallyneutral, are not affected by the strong electric field and pass throughthe region defined by screen 16 and receptor sheet 81. As coronagenerated ions pass through the screen, these ions interact with theaerosol, charging the aerosol particles which are subsequently drawnonto the receptor sheet.

While the aerosol generation embodiment shown in FIG. 2 involves the useof an air gun type atomizer 80, the invention is not restricted to thisgeneration technique. Other means of forming a jet include directlyatomizing a liquid through fine jets or thermally vaporizing a materialto form an aerosol cloud.

In addition to using either a liquid aerosol or a thermally vaporizeddyestuff, aerosol development, employing a solid powder, the so-calledpowder cloud development may be employed. Methods for generating powderclouds and details of powder cloud development are described in Dessauerand Clark Xerography and Related Processes, pages 309 through 340. Animportant difference between the use of a powder cloud in the presentinvention and powder clouds associated with conventional electrostaticphotography involves the fact that, in the process of the presentinvention the aerosol powder cloud should be uncharged or the charge perparticle should be maintained at a rather low value.

Dyestuffs which maybe successfully vaporized from a hot surface to forma uniform aerosol cloud include Brilliant Oil Blue, Oil Brown 0, and OilBrown N.

FIG. 3 is a drawing of a modification of the apparatus shown in FIG. 1which enables the process to be employed with plain paper. Theelectrostatic image development, employing a liquid toner, occursessentially simultaneously with charging and exposing. A shallow metaltray 84, having rubber seals 86, contains a conventional liquidelectrostatic toner which is continuously recirculated through thesystem by inlet and outlet tubes 91 and 92, respectively. A plain paperweb 88 passes over the rubber seals 86. The liquid electrostatic tonerlevel is maintained so that it is in contact with the paper web. Thecorona modulating screen 16 is spaced between one-fourth inch and l inchabove the surface of the paper. The corona source, power supplies, andillumination source are similar to those shown in FIG. I and are notshown here.

Radiant heater 93 is provided for heating the paper, in order to driveresidual moisture from the paper, prior to its being employed in theprocess of FIG. 3.

If conventional liquid electrostatic toners of the type employed in zincoxide paper machines are utilized in the apparatus of FIG. 3, it isfound that the paper picks up residual toner in uncharged areas, leadingto an overall grey background. This problem has been overcome bydiluting these commercially available liquid toners with carrier solventin an amount of 8 parts solvent to 1 part toner. At this dilution, thesolids content is near 0.1 percent. A majority of commercially availableliquid electrostatic toners employ aliphatic hydrocarbon solvents as theliquid carrier. Effective dilutions may, therefore, be carried outemploying the aliphatic hydrocarbon solvent lsopar G, manufactured byHumble Oil & Refining Company.

The spacing between the bottom of developing pan 84 and the lowersurface of web 88 is critical. If the spacing is too little,insufficient density is developed in the image, while if the spacing istoo great, low density images are also observed. Optimum spacings appearto range from 0.050 inch to 0.300 inch. Optimum results are obtainedunder conditions such that the exposure time is short, generally onesecond or less. These conditions are realized by employing anillumination intensity at the corona screen of foot-candles or greaterand high corona current which is obtained by running the corona wires athigh potentials and spacing the wires reasonably close to control screen16.

In certain highly absorbent papers, 21 background image is observed evenat low toner dilutions. This is caused by the takeup of developerparticles into the surface of the paper as the toner is absorbed by thepaper. This background may be eliminated with the addition of auxiliaryroller 87 which supplies a pure aliphatic hydrocarbon solvent to the webprior to the web contacting the liquid developer. The solvent, typicallylsopar G, is fed to roller 87 as this roller revolves through a pan 89containing the solvent. Since the web is already saturated or prewetwith pure solvent, no developer takeup occurs in the paper, thusresulting in cleaner backgrounds.

It has been found that, to a first approximation, the density of a tonedimage is roughly proportional to the charge per unit area that isdeveloped. A charge density of approximately 0.15 lcouL/cm is requiredto develop a dense image. The potential to which a charge supportingmember must be charged in order to develop this charge density isinversely proportional to the capacity of the per unit area of thechargeable member. Dielectric papers, having a dielectric coatingthickness of approximately 6 microns, develop dense images when chargedto potentials of 300 volts, corresponding to a charge density close to0.15 ucouL/cm Ifthe charge is developed across a 3 mil sheet of paper,the surface must be charged to potentials in the region of 3,000 to4,000 volts to obtain this charge density. In view of this requirement,it has been found necessary to employ high potentials between screen 16and developer container 84. Minimum potentials of 15 kv are requiredwith kv resulting in higher resolution images having less distortion. Ithas further been found that the developed due to the low capacity perunit area of paper compared to the thin dielectric coatings utilizedwith dielectric coated paper.

FIG. 4 illustrates yet another apparatus employing a photoconductivecoated screen together with development apparatus to generate a visibleimage employing plain paper. Inthis figure, screen 16, power supplies,illumination source, etc., are similar to FIG. 1. A paper web 98 issupported by paper drive rollers (not shown) so as to be spaced a veryslight distance above conducting roller 94. This roller serves in amanner similar to paper backing plate 10 of FIG. 1, and is electricallyconnected to power supply 21. Roller 94 revolves, the lower surfacepassing into tray 95 containing an ink 96 dispersed or dissolved in apolar liquid. During operation, roller 94 revolves carrying up a thinfilm of ink over its surface. The roller and paper web speeds areadjusted so that the web surface speed is equal to the velocity of theperiphery of roller 94. Since this is a dynamic process, means areprovided for moving the image from left to right across screen 16 at thesame velocity as the paper moves from left to right. Thus there is speedcorrelation between the image projected on screen 16, paper web 98, andthe motion of the ink film on roller 94 directly below the paper. Assurface charge develops on the upper surface of paper web 98, an intenseelectrostatic field is developed between the paper and conducting roller94. The high electrostatic forces generated in the gap between thebottom of the paper and the ink film cause the ink to jump from theroller to the surface of the paper, thereby forming a permanent visibleimage. A wide variety of inks are effective in this process, includingalcohol and water base inks consisting of colloidal carbon dispersions,opaque dye pigments, or dissolved acid or basic dyes. For effectiveoperation, the gap spacing between the ink film and the lower surface ofthe paper must be maintained uniform and, for typical operatingconditions, between the extremes of 2 mils and 50 mils. An operating gapof 5 mils appears preferable. With certain inks and at certainvelocities, it is difficult to establish a uniform ink film thickness onthe surface of roller 94. In this event, thickness control attachmentswell known to the art, such as doctor blades or reverse rolls, may beadded to establish the proper ink film thickness. This'process has theadvantage, in addition to using ordinary paper, of generating anextremely clean background since no ink or developer touches the paperin areas which are not charged. For certain papers, under high humidityenvironmental conditions, the paper web must again be preheated so thatthe charge placed upon the surface of the paper does not diffuse withina period of a few tenths of a second. This process functions effectivelywith standard weight plain papers since very high potentials, on theorder of 2,000 to 3,000 volts, are placed on the surface paper and thespacing between the top surface of the paper and conducting roller 94 isonly a few thousandths of an inch. These high potentials establishedacross such a short distance result in high electrostatic forces beingdeveloped at the surface of the ink film; such forces being sufficientto locally draw the ink film across the printing gap.

FIG. illustrates a further means of employing a corona currentmodulating screen to realize simultaneous charging, exposing, anddeveloping while using a plain paper. Endless conducting belt 108,supported and driven by conducting rollers 110, is employed to support alayer of developer or toner particles and is positioned, with uniformspacing, immediately below paper web 88. The potential of the conductingrollers and the conducting endless belt is established by power supply21. A uniform thin film of dry toner particles is continuously suppliedto the endless belt from hopper 112 containing a reservoir of tonerparticles 114. After transversing the development area, toner particlesfalling off of the endless belt are collected, for reuse, in tray 116.The operation of the apparatus shown in this figure is similar to thatof FIG. 4. The motion of the endless belt and paper may be continuous;in which case the image to be reproduced must be scanned across screen16 to match velocity of paper web 88, or the machine may operate in astep and repeat mode; the paper and endless belt advancing betweensuccessive exposures. Either a conducting or nonconducting toner may beemployed with this apparatus; the toner being transferred from theendless belt to the underside of the paper web by virtue of the highelectrostatic forces existing at charged regions of the paper. The sameprecautions regarding the endless belt paper spacing indicated forapparatus of FIG. 4 are pertinent for this apparatus.

FIG. 6 schematically illustrates an apparatus employing a coronamodulation screen 16 in such a manner as to form a visible toned imageon a plain paper sheet 120 which is supported on backing plate 10. Theimage projection source, corona wire, screen, and backing plate areidentical to the apparatus as shown in FIG. 1, and are connected topower supplies as shown in FIG. 1. In this device, charging and toningof the paper image are carried out simultaneously by employing an openmesh screen 122 moving through toner reservoir 130. The open mesh screen122 consists of a fine mesh (generally I00 to 300 meshes per inch)formed as an endless belt and traveling over drive pulley 124, idlerpulley 126, and pulley 128 which carries the open mesh through a tonersupply 132. In operation, the open mesh web is driven through toner toprovide a toner laden mesh surface immediately adjacent the paper uponwhich the image is to be developed. During an exposure, ions passingthrough the ion corona current modulating screen impinge upon the openmesh screen carrying toner, charging the toner particles to a highpotential, and the toner particles are subsequently electrostaticallyattracted onto the plain paper sheet 120. Toner is thus deposited on thepaper in areas corresponding to regions in which corona currenttraverses modulating screen 16. Either liquid or dry toners may beemployed in this apparatus, although best success has been realized withthe use of dry toners. The toners are not, in general, electrostaticallyheld onto the mesh screen 122 but are collected mechanically. It hasbeen found advantageous, in stances where low toner pickup on the openmesh endless belt 122 is observed, to very slowly move the belt duringthe exposure. This provides an additional toner source as toner isdepleted from the belt. Optimum belt drive speeds are in the range ofone thirty-second inch to one-half inch per second.

FIG. 7 illustrates schematically an apparatus for obtaining full colorprints employing the techniques of this invention. One of the majorproblems in generating full color prints, employing color separationprinciples, is associated with registration of the three colors. A minorproblem involves the complexity and expense in handling the copy sheet.The apparatus of FIG. 7 circumvents these difficulties by sequentiallygenerating a charge image pattern and developing the three primarycolors without moving the charge receptor layer. In this figure, theprojector 22 serves to project a color transparency onto the coronamodulation screen 16. Three successive exposures are provided; one forblue, a second for red, and a third for green. The projected color isselected by placing a filter color wheel containing the three selectedcolor filters 50 between the projection lens and the screen. Thesequential operation of the three primary colors is carried out byindexing motor 52. The charge receptor sheet is developed in placeemploying a series of three liquid toners which are delivered in such amanner as to flow over the receptor sheet from manifold 54. Solenoidactivated valves 56 select the appropriate yellow, cyan or magentaliquid toners which are contained in gravity fed reservoirs 58. Theliquid developed, after passing across the surface of the chargereceptor sheet, is collected in reservoir 60 and discarded or elsereclaimed for further use.

In operation, the blue filter is indexed in front of projector 22 sothat the image corresponding to the blue tones in a color print areprojected upon modulating screen 16. The required potentials are appliedto the corona and backing plate to form a charge image on the receptorsheet, and the solenoid valve connecting the yellow toner reservoir toapplicator manifold 54 is opened for a period of 2 to 4 seconds. Thetoner passing over the inclined surface of the charge receptor sheetdevelops the yellow components of the image. This process is thensuccessively repeated with a red color filter using a cyan toner and thegreen color filter employing a magenta toner. Effective liquidelectrostatic colored toners for use in this apparatus are manufacturedby the Day-Glo Corporation (Cleveland, Ohio).

An unexpected result obtained with this apparatus is the lack of arequirement for drying the charge image receptive paper betweensuccessive exposures. An image may be toned and, before the paper isdried, a second image placed on the surface.

When excess liquid remains at the surface of the charge image receivinglayer after a developer has flowed over the surface, this excess liquidmay be removed by drawing a rubber squeegee or rolling a hard rubberroller over the surface of the paper. Alternately, excess liquid can beremoved from the surface with the use of an air knife.

The latent electrostatic image formed by the corona modulation screenmay also be employed in recording crystal films in display applications.Here, the dielectric coated paper of FlG. l is replaced by a liquidcrystal film and the support platen 10 replaced by a glass sheet havinga transparent conductive coating on the side adjacent the film. Underthe influence of an electric field provided by ions reaching the freesurface of the liquid crystal film, the optical scattering and/orreflective properties of said film are modified, leading to theformation of a visible display on the film. Cholesteric materialssuitable for this application are described in British Pats. Nos.1,123,117 and 1,167,486, and also by L. Melamed and D. Rubin, Appli.Phys. Lett. 16, 4, 149 (1970) and by J. .l. Wysocki, .1. Adams, and W.Haas, Phys. Rev. Lett. 20, 19, 1024 (1968).

The following examples illustrate the techniques of the method, processand apparatus described in this disclosure. These examples are not meantto be restrictive in any way however.

EXAMPLE 1 A plain square weave 400 mesh stainless steel screen wasstretched over a square brass frame whose inside dimension was 6 incheson a side and whose outside dimension was 7 inches. The screen was softsoldered onto the frame. The frame was mounted in a vacuum coated anaverage distance 12 inches from a quartz crucible mounted in tantalumheater. The screen was inclined 45 from the normal. A charge of 30 gramsof high purity selenium was placed in the evaporation crucible. Thesystem was evacuated to a pressure of 10' torr-and the seleniumevaporated from the boat onto the screen over a period of 45 minutes.During the I evaporation, the screen was heated, with an electricalheater, to a temperature of 90C. The selenium coating thickness wasfound to be 20 microns.

The screen was removed from the vacuum evaporator and mounted in'theapparatus shown in FIG. 1. A 6 inch corona wire comprised of a 2 milthick diameter platinum wire was supported a distance of 1 inch abovethe screen. The screen to conducting platen spacing was one-half inch.

The contrast ratio, defined here as the ratio between the ion current tothe conductive backing plate with the photoconductive screen in the darkand ion current with the same screen illuminated, was determined byconnecting a Keithley Model 600A electrometer between an electricallyisolated portion of paper supporting electrode 10 and power supply 21.At a counterelectrode potential of 5 kv and a corona potential of +16kv, the dark current was 24 tamperes and the current obtained when thescreen was uniformly illuminated with tungsten illumination at a levelof 10 footcandles was 0.3 uamperes. The contrast ratio was thus 80. At acorona potential of +12 kv, the dark current was 1 1 uamperes and thelight current was 0.15 uamperes; yielding a contrast ratio of 75.

It may beseen from the aforementioned measurements that a highercontrast potential is obtained at lower corona potentials. In thisevent, however, the corona current is lower, and longer exposure timesare required to charge the dielectric paper. At a screen-paperseparation of one-half inch, an applied potential of 3 kv is sufficientto accelerate the ions to the surface of a dielectric coated paper andstill maintain a resolution of 6 line-pairs/mm in the developed image.

Copies of a projected image were obtained by placing sheets ofdielectric coated paper on the paper counterelectrode 10. An imagehaving a high-limit brightness of 10 foot candles was projected on thescreen with a simultaneous application of corona and counterelectrodepotentials; the total exposure time being 2 seconds. The paper was thenremoved from the'counterelectrode and immersed in a beaker of liquidelectrostatic toner having a solids concentration of 1 percent. Sincethe paper surface was charged positively and since the liquid developertoner particles are also positively charged, a reversal image wasobtained. When a positive line copy image was projected on the screenand the resulting latent electrostatic image developed in liquid tonerwith negatively charged particles, then the developed image was apositive image, i.e., black characters on a white background asexhibited bythe positive original. After the paper was removed from thedeveloper, excess liquid was squeegeed from the surface and the paperdried in an air stream which may be heated. The image was of highquality, having negligible background and a maximum density of 1.1. Thedevelopment time was 3 seconds.

EXAMPLE 2 Apparatus as shown in FIG. 2 was assembled. The backingplate-photoconductor coated screen-corona projection source, shown inFIG. 1, was used in this example with the screen and backing platemounted in a vertical direction.

A stainless steel 400 mesh screen was coated with a 20 micron layer ofselenium by vacuum vapor deposition. During the coating operation, thescreen temperature was maintained at C and the screen was angled suchthat the coating was deposited upon thescreen at an angle of 45 to thenormal.

The apparatus of FIG. 1 was further modified to include one additionalpower supply and a means for generating an aerosol which would traverseacross the face of a receptor sheet during an exposure, thus providingfor simultaneously charging, exposure, and development. By this means itis possible to employ ordinary plain paper as a receptor sheet forcharged aerosol particles. No further processing is required other thanthe heating of the receptor sheet to fix the toned image. A number ofapproaches were employed for generating a neutrally charged aerosolsuitable for employment in this example. a

One technique involved the use of a nichrome wire 15 mils in diameter.This wire was precoated with a film of DuPont Oil Brown 0, adark-brownanthraquinone dye. The wire was positioned midway between thephotoconductive screen and the receptor sheet near the bottom of thescreen. During the exposure and while potentials were applied to thecorona wires and the paper conductive backing, current was passedthrough the nichrome wire, raising its temperature just below red heat.The anthraquinone dye rapidly volatized without decomposition from thewire and, because of the thermal currents generated, traversed the spacebetween the screen and the plain paper receptor sheet. The potential ofthe wire, neglecting the small potential drop required to heat the wire,was at ground. It was found that positive ions traversing the screen inunexposed areas deposited a charge upon the aerosol, resulting in theformation of a visible print as the oil dye deposited upon the receptorsheet due to the electrostatic forces present. I

In another modification, an aerosol of dry powder was generatedemploying a distributor manifold having small apertures and extendingacross the bottom of the opening defined by the photoconductive coatingscreen and the plain paper backing electrode. A number of both wet anddry aerosols were employed in this apparatus, pressurized gas drivingthe aerosol through the distributor manifold. Carbon blacks, coloredpigment particles, and both conducting and nonconducting inks wereemployed. The potential of the pigment particle system, as shown in FIG.2, is controlled by power supply 83. Depending upon the triboelectriccharging properties, this potential was adjusted to minimize backgrounddensity.

In additional experiments, the power supply output was not connecteddirectly to the conductive manifold (which was maintained at groundpotential) but was connected to a bar electrode mounted immediatelyabove and to the side of the aerosol ejection manifold. Conductiveaerosols, ejected through the distribution manifold ports, were thuscharged by induction; the potential of the particles depending upon thepower supply 83 potential.

EXAMPLE 3 The apparatus of FIG. 3 was assembled. The corona modulatingscreen 16 was employed to modulate the corona current to a continuousreceptor web 88. The corona wire assembly, means for moving the screen,etc., were identical to that employed in previous examples. The liquidtoner reservoir 84 was 5 inches square and contained inlet and outlettubes 91 and 92 through which a 1 percent solids content, conventional,electrostatic toner was circulated. A large O-ring seal 86 was cementedto the top of toner reservoir 84 to confine the liquid toner to theregion immediately below web 88.

Excellent results were obtained in a step and repeat mode using a 3 milpolyester film as web 88. In the case of the polyester film, preheatingand presolvent wetting was not required. Exposure times, at a screenillumination intensity of foot-candles, ranged from one-half to 2seconds.

EXAMPLE 4 The apparatus as shown in FIG. 4 was assembled. Thephotoconductive screen of Example 1 was employed in this modification. Asingle corona wire, 4 inches in length and 2 mils in diameter, waspositioned l inch above the photoconductive coated screen 16. Thephotoconductively coated side of the screen was positioned up. Thespacing between photoconductive screen 16 and plain tape web 98 wasone-fourth inch. A solid brass roller 94, 2 inches in diameter, wasemployed to carry ink to a position immediately adjacent but nottouching the paper web. This roller 94 revolved through a trough of inkwhich was formulated by dissolving 2 percent (by weight) crystal violetdye in water. The apparatus was adjusted so that a gap of 10 mils waseffected between the surface of the ink covered roller and the bottom ofthe paper web 98. In operation, the paper web was run at speeds ofone-fourth to 3 inches per second while the brass roller was revolved sothat the peripheral speed of the brass roller was identical to thevelocity of the paper web. At the same time, a positive optical imagewas projected onto the screen and caused to move across the screen atthe same velocity and in the same direction as the paper. Thus, therewas no relative motion between the optical image and the paper. Duringoperation, the brass roller 94 was maintained at ground potential, thephotoconductively coated screen 16 was maintained at +5 kv, and thecorona wire was operated at a potential of +20 kv.

At an unilluminated region of the screen, corona current was transmittedto the top surface of the plain paper and the intense local field causedthe ink to jump from the roller onto the back surface of the paper.Effective printing operation was obtained at paper web velocities up toI inch per second.

EXAMPLE 5 In this example the apparatus of FIG. 4 was again employed.One modification involved replacing the aqueous ink developer with acommercially available electrostatic toner in this case a Hunt ChemicalCorporation reversal electrostatic toner. This toner was diluted, overthe recommended concentration generally employed in electrostaticdevelopment, by mixing 1 part reversal toner to 8 parts of lsopar G, ahydrocarbon solvent manufactured by Humble Oil & Refining Company. Asecond difference from the experiments of Example 4 involved reducingthe paper to roller spacing so that a liquid developer meniscus wasformed between the brass roller and the bottom of the paper web. Again,during operation, developed images in unexposed areas were obtained atweb velocities approaching 2 inches per second. It was found possible inthis example to run the brass roller at angular velocities such that theperipheral speed of the roller was several times the paper velocity.

' EXAMPLE 6 The apparatus of FIG. 5 was assembled. The same screen,corona wire, applied potentials, optical scanning system, etc. that wereemployed in Example 4 were utilized. Two brass rollers were employed tosupport a 400 mesh stainless steel screen formed into a continuous belt.During operation, the spacing between the plain paper web 88 and thesurface of screen 108 was maintained at a distance of approximately 20mils. Dry electrostatic toner powder 114 was continuously supplied ontothe endless belt from a hopper spaced directly above the screen over oneof the brass rollers. During operation, positive images were obtained atweb velocities up to 2 inches per second. It was found in that the drytoner would sometimes fall off the web after the web had passed throughthe toner deposition region. This was minimized by preheating the paper,using a blast of hot air from a hot air gun to a temperature ofapproximately C. This served the function of partially fusing the tonerto the paper surface almost immediately after the toner was depositedupon the surface.

EXAMPLE 7 The apparatus of FIG. 2 was modified with the addition of acontinuous belt composed of a fine mesh screen which was mechanicallysupported so as to be driven across the surface of the paper as shown inFIG. 6. The screens evaluated were formed into a continuous belt 5inches wide and 20 inches long. The screen was driven by a metalcylinder drive roller 124, 2 inches in diameter. Idler rollers 126 and128 positioned the screen and provided support for the screen as thescreen was driven through toner 132 contained in toner reservoir 130.Samples were prepared under a variety of conditions ranging from thecase in which a screen was stationary in front of the paper during anexposure to situations in which the screen was driven at speeds to 2inches per second past the surface of the paper during the exposure.Both metal, phosphor bronze, and stainless steel as well as dacron andnylon screens were evaluated; the screen mesh sizes ranging from 200mesh to 325 mesh. it was found that both the nonconducting andconducting screens were equally satisfactory informing imagescorresponding to the image projected onto the corona modulating screenon the plain paper sheet 120.

In operation, the corona modulating screen was maintained at groundpotential and the plain paper support platen was maintained at l kv. Anumber of experiments were carried out in which thetonercontaining-screen to paper spacing was varied, and no substantialdifference in image quality was found over spacings from a few mils toone-eighth inch.

Both conventional liquid electrostatic toners and dry carbon powderswere employed satisfactorily in this apparatus. A number of finelydivided carbon particles were evaluated and excellent results wereobtained using Van Dyke Corporation gold seal toner. The lightintensities and exposure times required were, in most cases, similar tothat of Example 5.

in the case of the metal screen, good results were obtained when thescreen was operated at potentials in the region of to 8 kv. Equally goodresults were obtained when the screen-roller-toner reservoir assemblywas left floating, i.e., not connected directly to any potential. inthis case, the screen assembly probably was stabilized to a potentialnear -l0 kv due to capacitive coupling between the screen and papersupport platen 10.

EXAMPLE 8 The apparatus of FIG. 1 was modified by tilting the wholeapparatus at an angle of 30 to the horizontal and addinga developermanifold, solenoid operated valves and developer supply tanks as shownin FIG. 7. In addition, provisions were made for placing color filtersin front of the slide projector 22. The developer manifold consisted ofa /4 inch diameter copper tube (6 inches long) sealed at one end. Holes0.020 inch in diameter were drilled in a line along the distributionmanifold at a spacing of one-fourth inch. The open end of the copperdistribution manifold was connected to electrically operated solenoidvalves 56 with suitable unions. Each of the three solenoid valves wereconnected in turn to a liquid developer reseroir. A collection pan 60was provided to collect the liquid developer after it had flowed overthe surface of the paper being developed.

A full color positive transparency was placed in projector 22 and ayellow filter placed in front of the projector. The high-light intensityat the screen was 20 placed on the brass paper support platen.Immediately after the yellow development operation was completed, thered filter was placed in front of the slide projector. The lightintensity was increased until a high-light intensity of 60 foot-candleswas incident upon the screen and the image exposed with the potentialsapplied to screen and the image exposed with the potentials applied toscreen and backing electrode for a period of 1 second. The solenoidvalve connecting the developer manifold to the cyan liquid tonerreservoir was opened again for a period of 2 seconds. The process wascontinued a third time employing a green filter with a highlightintensity of 20 foot-candles and a 1 second exposure followed by a 2second magenta development. The paper was then removed from theconducting brass platen and dried in a warm air stream. A positive printwas thus obtained having a good color balance and a high degree ofregistration.

in several runs it was found that the charged colors had run somewhat.This problem was eliminated by removing excess liquid developer from thesurface of the paper after each development operation. It was found thatthis liquid removal could be effectively carried out by employing eithera rubber squeegee, a /1 inch diameter rubber roller which was run overthe dielectric paper, or by directing a high velocity jet of air overthe surface of the paper to blow excess developer from the papersurface.

While but a limited number of embodiments of the present invention havebeen here' disclosed, it will be apparent that many variations may bemade therein without departing from the spirit of the invention asdefined in the following claims.

We claim:

1. In an apparatus for preparing visible images on a receptor sheetconforming to and simultaneous with an optical image projected onto anion-permeable member having a photosensitive coating thereon whichincludes:

on ion source;

a photosensitive coated ion-permeable member;

an image receptorsheet;

optical image projection means;

an open mesh web adapted to transport neutral electrostatic tonerparticles, said web being positioned between said image receptor sheetandsaid ionpermeable member; and

means for supplying electrical potential between said ion source andsaid ion-permeable member, and means for supplying electrical potentialbetween said ion-permeable member and said image receptor sheet duringexposure said electrical potentials serving respectively (1) to directions which traverse the ion-permeable member onto the toner laden weband (2) to direct toner particles onto said image receptor sheet.

2. The apparatus of claim 1 includes a toner supply through which saidopen mesh web is passed and then caused to move past the surface of saidimage receptor member.

3. The apparatus of claim 2 wherein said open mesh web is electricallyconductive.

4. The apparatus of claim 2 wherein said open mesh web consists of awoven screen of synthetic polymer filaments.

5. The apparatus of claim 2 wherein said toner conthereon.

1. In an apparatus for preparing visible images on a receptor sheetconforming to and simultaneous with an optical image projected onto anion-permeable member having a photosensitive coating thereon whichincludes: on ion source; a photosensitive coated ion-permeable member;an image receptor sheet; optical image projection means; an open meshweb adapted to transport neutral electrostatic toner particles, said webbeing positioned between said image receptor sheet and saidion-permeable member; and means for supplying electrical potentialbetween said ion source and said ion-permeable member, and means forsupplying electrical potential between said ion-permeable member andsaid image receptor sheet during exposure said electrical potentialsserving respectively (1) to direct ions which traverse the ionpermeablemember onto the toner laden web and (2) to direct toner particles ontosaid image receptor sheet.
 2. The apparatus of claim 1 includes a tonersupply through which said open mesh web is passed and then caused tomove past the surface of said image receptor member.
 3. The apparatus ofclaim 2 wherein said open mesh web is electrically conductive.
 4. Theapparatus of claim 2 wherein said open mesh web consists of a wovenscreen of synthetic polymer filaments.
 5. The apparatus of claim 2wherein said toner consists of finely divided carbon particles having nocharge thereon.